Jet fuel is a hydrocarbon boiling in the 350 to 572° F. range. In addition to constituting the power sources for gas turbine engines used in both ground-based and military and civilian aviation applications increasing demands are being placed on the fuel, as aircraft evolve, to function as a coolant/heat sink for engine and other equipment; i.e., aircraft subsystems. Consequently, jet fuel is exposed to temperature environments substantially hotter than traditionally encountered when used simply as a fuel.
By the exposure of the fuel to such higher temperature environments, such as the system for cooling aircraft engine subsystems or engine lubricant oils, the jet fuel is subjected to heat induced stress which causes fuel thermal oxidation breakdown products to form; e.g., gums, lacquers, coke, ash, which can and do form deposits on engine internal parts leading to engine inefficiency and, in extreme cases, engine failures. This situation leads to reduced maintenance intervals and significantly increased maintenance costs.
To combat such thermal oxidation breakdown, fuel formulators have begun adding enhanced thermal stability additive to the fuel, which slow the reaction of the fuel hydrocarbon components with the dissolved oxygen in the fuel and disperse those polymeric oxidation products which do form so that they pass through the engine and burn during combustion rather than accumulating and depositing on engine component surfaces such as fuel controllers, burner nozzles, the afterburner spray assemblies, the manifolds, the thrust vectoring actuators, the pumps, the valves, the filters and the heat exchanger surfaces. Engine smoke emissions and noise also increase as a result of the thermal-oxidative deposits.
Numerous additives and additive systems have been put forward for the enhancement of the thermal stability of hydrocarbon materials.
WO 98/20990 discloses a method for cleaning and inhibiting the formation of fouling deposits on jet engine components. The method involves the addition of a derivative of (thio)phosphenic acid to the jet fuel. Unfortunately, the (thio)phosphenic acid disarms the filters in the ground-based water-separators. Therefore, this additive must be added to the jet fuel at the skin of the aircraft; i.e., this additive must not be added to the jet fuel prior to fuelling the aircraft.
WO 99/25793 discloses the use of “salixarenes” to prevent deposits in jet fuel at a temperature of 180° F.
U.S. Pat. No. 5,468,262 discloses the use of phenol-aldehyde-polyamine Mannich condensate with a succinic acid anhydride bearing a polyolefin to improve the thermal stability of jet fuel at 260° F.
U.S. Pat. No. 3,062,744 describes the use of a hydrochloric acid salt of a polymer formed from an amine-free monomer and an amine-containing monomer for reducing deposits in refinery heat exchangers. It is stated that polymer itself is not effective, only the HCl salt.
U.S. Pat. No. 2,805,625 relates to the stabilization of petroleum-based oils in storage. Polymers of amino-containing monomers with oleophilic monomers were found to be ineffective for demulsifying water-oil mixtures. Water separation was achieved by adding a further co-additive of a fatty acid amide.
GB 802,588 describes a fuel composition comprising a copolymer of a compound with at least one ethylenic linkage and at least one α-β-unsaturated monocarboxylic acid. The acid monomer may be derivatized with polar groups provided that at least 20% of the carboxyl groups remain unreacted.
Because jet fuel is also exposed to lower temperatures during use that cause free water present in the jet fuel to freeze, which can cause plugging of filters and other small orifices, and occasionally engine flameout, such free water must be removed from the fuel prior to delivery to the end user, be it commercial or military. As jet fuel is transported through the distribution system (i.e., pipelines, ships, barges, storage tanks, etc.), it can pick up free water from the drop out of dissolved water when the fuel cools, condensation of atmospheric moisture and ground water/rain water incursion. This water is normally removed by passing the jet fuel through filter/coalescer and separator systems, such systems being comprised of a filter/coalescer cartridge and a separator cartridge specified by API/IP 1581 3rd edition or 5th edition (Category C) at several points in the fuel distribution system, usually at least into and out of airport storage facilities. Military and certain FSII (fuel system icing inhibitor also known as diethylene glycol monomethyl ether (DiEGME)) users may use API/IP 1581 5th edition Category M or M100 filter systems but the use of these systems is generally limited to the end of the distribution system. Into-plane jet fuel water content standards are either 15 ppm (ATA-103) or 30 ppm (IATA) as cited in the airline operator's handling standards, where ATA-103 is commonly cited in the U.S. and IATA ex U.S. (outside the former Soviet Union and China). These limits are always met when FSII is absent and properly operating API/IP 1581 filter systems are used to filter Jet A or Jet A-1 for commercial aviation. (The Jet A/A-1 international specifications, D1655 and DefStan 91-91, limit the formulations and concentrations of additives to protect the water separability performance of API/IP 1581 filter systems.) The maximum effluent water content permitted by API/IP 1581 in laboratory compliance testing is 15 ppm. It has been found that fuels additized with certain various additives, particularly with thermal stability additives, degrade the water removal performance of API/IP 1581 filtration systems, so that the filtered fuel may not be sufficiently is dry to meet into-plane water content standards. Such additized fuels are considered to be not “filter friendly” and cannot be distributed via the existing API/IP 1581 compliant distribution system without significant modifications. Currently the use of such additives is limited to military fuel (e.g. JP-8) and non-commercial use of Jet A/A1.
EP 1,533,359 teaches a thermal-oxidation stability additive comprising one or more copolymer, terpolymer or polymer of an ester of acrylic acid or methacrylic acid or a derivative thereof wherein the copolymer, terpolymer or polymer of an ester of acrylic acid or methacrylic acid or derivative thereof is copolymerized with a nitrogen-containing or amide-containing monomer, or the copolymer, terpolymer or polymer of an ester of acrylic acid or methacrylic acid or derivative thereof includes nitrogen-containing, amine-containing or amide-containing branches. The additive package containing this material also preferably contains at least one aminic or phenolic acid, or both, at least one ashless dispersant, preferably a hydrocarbyl or polyalkenyl succinimide or a derivative thereof. Other optional additional components can include metal deactivators, lubricating additives, corrosion inhibitors, anti-icing additives, brocides, anti-rust agents, anti-foaming agents, demulsifiers, detergents, cetane improvers, stabilizers, static dissipaters and the like and mixture thereof. It is reported that the additives system does not adversely affect the API/IP 1581 water coalescing filters which form part of the ground-based fuel delivery system, on the basis that the additive gives passing MSEP (ASTM D3948) results. While MSEP is widely used in the aviation industry to control the content of natural surfactants in jet fuel that are known to degrade the water separation performance of coalescing filters, the materials and fuel flow rates of the MSEP test are sufficiently different from the field implementation of API/IP 1581 filter/coalescer and separator systems that MSEP cannot predict the performance of field systems.
Part of the deficiency of the MSEP evaluative process is that there are at least three mechanisms by which surfactants can inactivate (disarm) the water removal performance of API/IP 1581 filter/coalescer and separator systems:                1. Surfactants can reduce the interfacial tension between jet fuel and water stabilizing the persistence of very small water droplets. The small water droplets can move with the flow of jet fuel through the coalescer more readily than larger droplets and avoid being intercepted by hydrophilic fibers, which normally accumulate and coalesce small water droplets into larger, readily separable water droplets. In addition, if the water droplets are intercepted and coalesced by the fibers, the low fuel/water interfacial tension tends to cause the droplets to be redispersed as they pass through higher shear regions of the filter/coalescer cartridge.        2. Surfactants can adsorb on the hydrophilic surface of the coalescer media rendering it hydrophobic. The modified surface does not attract water droplets and thus the water does not coalesce.        3. Surfactants can adsorb on the hydrophobic parts of the coalescer media rendering it hydrophilic. The proper function of the coalescer media relies on nodules or nodes of hydrophobic material on the hydrophilic fibers to cause coalesced droplets to separate from the fiberglass surface when they reach a certain size. When these nodes become hydrophilic, coalescence is not limited, thus resulting the formation of sheets of water between fiberglass fibers. When these sheets become large enough, the flow of jet fuel through the coalescer bed disrupts them forming many very small water droplets that are reentrained in the jet fuel.The coalescence media in Alumicel® MSEP cartridges is not the same and the flow/shear rate is much higher versus commercial filter/coalescer cartridge elements so the MSEP number does not necessarily predict water removal performance in the field. For example a certain diesel lubricity improver reduced the MSEP of a jet fuel from 98 unadditized to 85 with 100 ppm of the additive (70 MSEP with 200 ppm of additive). Fuels with an MSEP rating of 85 normally are considered to be filter friendly; that is, to not disarm coalescers. In a week-long laboratory experiment designed to test the field coalescers of API/IP 1581 systems using the same materials as the field API/IP 1581 systems and scaled from field flowrates to 100 ml/min, the coalescers failed with only 35 ppm of this additive in jet fuel despite having passed the MSEP test. In another example, it is commonly believed that MSEP over responds to certain weak surfactants such as the aviation approved conductivity improver Stadis 450, where API/IP 1581 filtration systems are not necessarily disarmed by fuels with low MSEP values. The DefStan 91-91 jet fuel specification recognizes this by specifying different limits for jet fuel MSEP at the point of manufacture depending upon the content of Stadis 450 (70 MSEP min. in the presence of Stadis 450 and 85 MSEP min. in its absence). This demonstrates the need for the method disclosed below to accurately assess the impact of fuels/additives on API/IP 1581 systems that comprise the basis of the jet fuel distribution system. Thus a need exists for determining the filterability of each jet fuel in API/IP 1581 systems.        
An advantage of the present method is that it can determine whether jet fuel, regardless of the additive or additive package present in such fuel and regardless of the water content in such fuel, can be processed so as to be delivered with acceptable water content upon delivery; i.e., have a water content upon delivery to the final consumer of about 15 ppm or less, and to identify the specific processing conditions and limits.
The method disclosed herein can be used to map any effluent water level, but 15 ppm is preferred, to ensure that field systems operate to the same standards used in design and qualification of API/IP 1581 filtration systems.
The present invention can enable the wider application of thermal stability additives by reducing the risk of an incident caused by inadequate water separation performance of API/IP 1581 filter/coalescer and separator systems.